This invention relates in general to gas sensors and in particular to a method for improving the selectivity of FET-based gas sensors such that interfering effects of cross-sensitivities are minimized.
Gas sensors that utilize and evaluate the change in work function of sensitive materials as a physical parameter have been experiencing increased interest recently. The reasons for this are the ability to operate such sensors with relatively low operating energy (low operating power), the economical production and construction technology of such gas sensors (low production costs), and a broad palette of gases that can be detected with this platform technology (high versatility). Numerous different detection substances can be integrated into such structures. Their construction and mode of operation are disclosed, for example, in German Patent Applications 19814857, 19956744, 19849932 and 19956806. A number of materials can be used for sensitive layers in such gas sensors.
The basic structure of these prior art work function gas sensors is shown in FIG. 2, which schematically illustrates the structure of the work function of such a sensor with FET readout, particularly an SGFET (suspended gate field effect transistor).
In the presence of the gas to be detected, an electrical potential that corresponds to the change in the work function of the sensitive layer (typically 50-100 mV) is developed on the sensitive layer which is coated on the bottom of the raised gate electrode. This potential acts on the channel of the FET structure and changes the source-drain current. The changed source-drain current is measured directly. Alternatively, the change in source-drain current is restored by applying an additional voltage to the suspended gate or to the transistor trough. The additionally applied voltage represents a readout signal that directly correlates with the change in work function of the sensitive layer.
A basic problem with all gas sensors including the type described above is their relatively limited selectivity. In other words, the sensors under some circumstances react not only to the target gas but also to other gases, which is called cross-sensitivity. The superimposed gas signals then lead in many applications to a situation in which the target gas concentration cannot be determined with adequate reliability from the sensor signal, since the sensor signal is distorted in an unacceptable amount by the cross-sensitivity. Up to now, it has been necessary to accept the distortion of the sensor signal. The distortion effect can be partially eliminated by intelligent signal evaluation adapted to the application, but this possibility is relatively limited for many applications. Alternatively, an additional sensor can be used that is sensitive specifically to the interfering gas and whose additional signal is used to compensate for the interference effect in an appropriate signal processor. However, this approach involves substantially higher system costs.
What is needed is an FET-based gas sensor having relatively reduced distortion of the sensor signal due to cross-sensitivity.